The present invention relates generally to semiconductor device manufacturing, and, more particularly, to a method and structure for reducing prior level edge interference with critical dimension (CD) measurement.
The fabrication of integrated circuits such as, for example, memory devices using large scale integration (LSI), very large scale integration (VLSI) or ultra large scale integration (ULSI) involves the placement of extremely complex electrical circuits on a single chip of silicon. A photolithography process is frequently used to transfer a microscopic pattern from a photomask to the silicon wafer surface of an integrated circuit. In particular, the process involves several iterations of individual reductions, wherein each individual reduction may introduce errors into the final pattern.
In order to meet the objective of increasing the density of memory cells or other logic components on a chip, semiconductor processing engineers continue to refine wafer processing methodologies. Of particular importance are the patterning techniques through which individual regions of the semiconductor structure are defined. In an effort to increase the number of components in the semiconductor structure, integrated circuit configurations have evolved into complex, three-dimensional topographies characterized by several layers of material forming patterns overlayed with respect to one another.
As device and memory cell dimensions continue to shrink, certain measurement parameters become increasingly important. For example, the requirement for overlay measurement accuracy continues to increase in order to compensate for processing inaccuracies. Since a typical photolithographic system uses a step-and-repeat and step-and-scan process to transfer the mask pattern onto the chip, each successive pattern must be properly aligned to the previously existing patterns. Otherwise, each individual pattern transformation may introduce alignment or overlay errors.
In addition to the overlay measurements, measurements of the critical dimensions (CD) of features of patterns formed within each level within a semiconductor device are also made. CD measurements are commonly implemented using different features and using different techniques from those used for measuring overlay. It is a common practice to perform separate critical dimension measurements for each pattern formed within a semiconductor device in addition to separate overlay measurements.
Presently, the in-line CD scanning electron microscope (SEM) is the “workhorse” toolset for critical dimension control and measurement. The high resolution of the device allows for the measurement and control of the lithography and etching processes during semiconductor manufacturing. CD SEMs, when operated at low voltages, typically only probe a small distance into the structure being considered. As a result, CD SEMs are good at detecting surface information. In general, measurements made by a CD SEM are not influenced by layers underneath the top upper most layer. This allows the CD SEM to provide measurements that are pertinent to current processing steps, without being subject to “noise” (i.e., interference from other features) from prior processing steps.
Unfortunately, for some types of processing steps, there are certain device features (for which the in-line CD SEM is responsible for measuring and controlling) that do not conform to this general rule. For example, when trying to measure vias after lithography and post etching in dual damascene processes, layer-to-layer interactions add significant noise to the CD measurement.
As is known in the art of dual damascene processing, a via is imaged within a trench that will eventually become the metal line to which the via connects an earlier formed metallization layer. During the via processing step, the primary concern lies with the critical dimension of the via in the resist. However, the underlying topography of the metal trench can interfere with the measurement of the via. Moreover, the variability in the size of the top layer via and the metal trench beneath the via, combined with variability in the alignment between the top layer via and the trench, may result in situations where the intended via measurement is obscured by the trench below.
Ideally, a via is printed in the middle of the trench; however, in actual practice, the via actually formed in the resist often intersects with the edge of the trench therebeneath. In this case, when the CD SEM scans to measure the via, the resulting signal that is actually processed contains information about both the via and the trench below the via. As a result, the CD SEM measurement algorithm cannot reliably distinguish between the topmost layer that is the subject of the intended measurement (i.e., the via formed in resist) and the previously formed layer therebeneath (i.e., the metal trench below). This situation leads to incorrect CD measurements which negatively affect process control and which can lead to other adverse consequences, including yield loss.
Also, for a post-etch via measurement step, the problem is somewhat similar but more intractable. The signal considered by the CD SEM contains information, not only about the intended via to be measured, but also about the trench above the via and the trench below the via. Moreover, mismeasurements at this stage of processing are even more deleterious, in that by the time the post-etch step is reached, there is no hope of reworking the wafer.